Remove chessboard_render_elements(), chessboard_process_attached() and chessboard_formatter_view()

In 6.x-1.x these were deprecated, having added _chessboard_render_elements(), _chessboard_process_attached() and _chessboard_formatter_view(). In 7.x-1.x the first two of the old functions just look completely useless and very broken, while the third may not be broken, but surely useless, especially if we remove the first two.